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S7L3. Students will recognize how
biological traits are passed on to
successive generations.
A.Explain the role of genes and chromosomes in the
process of inheriting a specific trait.
B. Compare and contrast that organisms reproduce
asexually and sexually (bacteria, protists, fungi,
plants, and animals).
C. Recognize that selective breeding can produce
plants or animals with desired traits.
DNA
• DNA is often
called the
blueprint of life.
• In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
Why do we study DNA?
We study DNA for
many reasons:
• its central
importance to
all life on Earth
• medical benefits
such as cures
for diseases
• better food
crops.
Chromosomes and DNA
• Chromosomes
are made up of
genes.
• Genes are made
up of a chemical
called DNA.
The Nucleus
• DNA is located in the
nucleus
The Shape of the Molecule
• DNA is a very
long molecule.
• The basic shape
is like a twisted
ladder or zipper.
• This is called a
double helix.
Nucleotides
One deoxyribose together with
its phosphate and nitrogen base
make a nucleotide.
O
O -P O
O
Phosphate
Nitrogen
base
O
C
C
C
O Deoxyribose
One Strand of DNA
• The backbone of
the molecule is
alternating
phosphate and
deoxyribose
sugar parts.
• The steps are
nitrogen bases.
phosphate
deoxyribose
bases
Four nitrogenous bases
DNA has four
different bases:
•
•
•
•
Cytosine
Thymine
Adenine
Guanine
C
T
A
G
Important
 Adenine and
Thymine always join
together
A -- T
 Cytosine and
Guanine always join
together
C -- G
Two Stranded DNA
• Remember, DNA
has two strands
that fit together
something like a
zipper.
• The teeth are the
nitrogen bases
but why do they
bond together?
Hydrogen Bonds
• When making
hydrogen bonds,
cytosine always
pairs up with
guanine,
• And adenine
always pairs up
with thymine.
• (Adenine and
thymine are shown
here.)
O
N
O
C
C
C C
N
C
Do Now!
• Where is DNA
located?
• What does it look like?
• What are its bases?
• Why do you think DNA
is located there?
DNA Synthesis/DNA Replication
• Step 1- DNA unwinds and unzips
• Step 2- Once the molecule is separated it copies
itself.
• The new strand of DNA has bases identical to the
original
What are the main differences
between DNA and RNA
1. Thymine is replaced by
the nitrogen base
Uracil.
2. Deoxyribose sugar is
replaced by Ribose
sugar.
3. DNA is double
stranded, while RNA is
a single strand.
RNA
• In RNA Thymine is
replaced by Uracil
• A-U (RNA)
• not
• A-T (DNA)
• IF the DNA strand is GTACCAGATTAGC
• What would the matching RNA strand be?
G TAC CAGATTAG C
C A U G G U C U AA U C G
Protein Synthesis
Codon
• The flow of
information from gene
to protein is based on
codons.
• A codon is a threebase word that codes
for one amino acid
• The flow of
information from gene
to protein is based on
codons.
Information Flow: DNA to RNA to
Protein
Transcription
• When a secretary
transcribes a speech,
the language remains
the same. However,
the form of the
message changes from
spoken to written
Transcription
• Transcription- RNA is
made from a DNA
template in the
nucleus.
• This type of RNA is
called messenger RNA
or mRNA
Transcription
• DNA is protected
inside the nucleus.
• mRNA carries the
message of DNA into
the cytoplasm to the
ribosome's
Translation
• To translate English
into Chinese requires
an interpreter.
• Some person must
recognize the words of
one language and
covert them into the
other.
tRNA Transfer RNA
• The cells interpreter
• tRNA translated the
three-letter codons of
mRNA to the amino
acids that make up
protein.
Translation
• Genetic translation
converts nucleic acid
language into amino
acid language.
S7L3. Students will recognize how
biological traits are passed on to
successive generations.
A.Explain the role of genes and chromosomes in the
process of inheriting a specific trait.
B. Compare and contrast that organisms reproduce
asexually and sexually (bacteria, protists, fungi,
plants, and animals).
C. Recognize that selective breeding can produce
plants or animals with desired traits.
Genetics
and
Heredity
History
• Genetics is the study of genes.
• Inheritance is how traits, or characteristics, are
passed on from generation to generation.
• Heredity is the passing of traits from parents to
offspring.
• Chromosomes are made up of genes, which are
made up of DNA.
• Genetic material (genes,chromosomes, DNA)
is found inside the nucleus of a cell.
• Gregor Mendel is considered “The Father of
Genetics"
Mendelian Genetics Vocabulary
•
•
•
•
•
•
•
•
Dominant traits- traits that are expressed.
Recessive traits- traits that are covered up.
Alleles- the different forms of a characteristic.
Probability- the chances/ percentages that
something will occur.
– Punnett Squares- show how crosses are made.
Genotype- the types of genes (Alleles) present.
Phenotype- what it looks like.
Homozygous genotype- two of the same alleles.
Heterozygous genotype- two different alleles.
Gregor Mendel
(1822-1884)
• Austrian Monk.
• Experimented with “pea plants”.
• Used pea plants because:
– They were available
– They reproduced quickly
– They showed obvious differences in the
traits
Understood that there was something that carried
traits from one generation to the next“FACTORS”.
Mendel's Plant Breeding
Experiments
Gregor Mendel was one of
the first to apply an
experimental approach to
the question of
inheritance.
For seven years, Mendel
bred pea plants and
recorded inheritance
patterns in the offspring.
Mendel’s Terminology
• True breeding (purebred): When the plants self-pollinate,
all their offspring are of the same variety.
• Hybridization: Mating, or crossing, of two varieties.
• Inbreeding: The crossing of two individuals that have
similar traits.
• Monohybrid cross: A cross that tracks the inheritance of a
single characteristic.
• P generation: True breeding parents.
• F1 generation: (first filial) Hybrid offspring of the P
generation.
• F2 generation: (second filial) Offspring from the selffertilization of the F1 hybrids.
Mendel’s law of Dominance
• In a cross of parents
that are pure for
contrasting traits, only
one form of the trait
will appear in the next
generation. Offspring
that are hybrid for a
trait will all have only
the dominant trait in
the phenotype.
Mendel’s law of segregation
• During the formation of gametes (ovum or sperm), the
two alleles responsible for a trait separate from each
other. Alleles for a trait are then “recombined” at
fertilization, producing the genotype for the trait of the
offspring.
• Mendel discovered this 1860.
• DNA was not discovered until 1953.
Mendel’s law of Independent Assortment
• Alleles for
different traits
are distributed
to gametes
independently
of one another.
Reproduction in Flowering Plants Review
•Pollen contains sperm
–Produced by the
stamen
•Ovary contains eggs
–Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
Mendel’s Experimental
Methods
•Mendel hand-pollinated
flowers using a paintbrush
–He could snip the stamens
to prevent self-pollination
–Covered each flower with
a cloth bag
•He traced traits through the
several generations
Mendel was fortunate he chose the Garden Pea
•Mendel probably chose to
work with peas because they
are available in many
varieties.
•The use of peas also gave
Mendel strict control over
which plants mated.
•Fortunately, the pea traits
are distinct and were clearly
contrasting.
To test the hypothesis, Mendel crossed true-breeding plants that had
two distinct and contrasting traits—for example, purple or white
flowers.
What is meant by “true breeding?”
Mendel cross-fertilized his plants by hand. Why is it important to control which
plants would serve as the parents?
For each monohybrid cross, Mendel cross-fertilized true-breeding
plants that were different in just one character—in this case, flower
color. He then allowed the hybrids (the F1 generation) to self-fertilize.
Following the Generations
P (Purebred
Parents)
TT x tt
F1 Results =
all Hybrids
Tt
F2 results =
3 Tall & 1 Short
TT, Tt, tt
Trait: Plant Height
Paternal Gametes
Sperm
Maternal
Gametes
Egg
Egg
Sperm
T
T
t
Tt
Tt
t
Tt
Tt
Probability and Punnett Squares
Punnett square: diagram showing the probabilities of the possible
outcomes of a genetic cross
1. What are the parent’s
genotypes?
2. From this monohybrid
cross, what is the
probability of the parents
producing a homozygous
recessive offspring?
3. What is the probability of
the parents producing a
tall offspring ?
Genotype versus phenotype.
How does a
genotype ratio differ
from the phenotype
ratio?
Seven Pea Plant Traits
• Seed shape --- Round (R) or Wrinkled (r)
• Seed Color ---- Yellow (Y) or Green (y)
•
•
•
•
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
• Flower color --- Purple (P) or white (p)
Mendel studies seven characteristics in the garden pea
Identify the correct vocabulary term for
each condition presented
These are the units
which make up
chromosomes.
Responsible for
inheritance of specific
characteristics
Eye color,
skin color and hair color:
They are controlled by genes.
Sperm and egg cells are both this type of cell.
Contain half the amount of DNA of normal diploid cells.
When a sperm and egg cell
fuse together,
they produce this.
We use this word to describe
cells which contain the full
complement of genetic
material. In humans this would be
46 chromosomes (23 pairs)
The different versions of genes
Alleles of a given
gene are identical
(can be either
dominant or
recessive
Alleles of a given
gene are not
identical
Plants and animals with useful or desired
traits are bred together to produce offspring
with those desired traits
The altering of the characteristics of an
organism by inserting genes from
another organism
Division of the nucleus to produce 2 daughter
nuclei which each has the same
number and kind of chromosomes as
the mother cell
Type of reproduction that involves
fusion of gametes
S7L3. Students will recognize how
biological traits are passed on to
successive generations.
A.Explain the role of genes and chromosomes in the
process of inheriting a specific trait.
B. Compare and contrast that organisms reproduce
asexually and sexually (bacteria, protists, fungi,
plants, and animals).
C. Recognize that selective breeding can produce
plants or animals with desired traits.
A concept that can be used to predict the results of a particular event
– Examples
• Chance of a specific team winning a sporting event
• Chance of a coin landing on heads in a coin toss
– Predicts what is likely to occur, not necessarily what will actually occur
Probability and Heredity
• Think back to Mendel’s
plant experiments
– He noticed that traits were
inherited in patterns
• For example, when he
crossed two plants that
were heterozygous for
stem height (Tt) he
noticed their offspring
would inherit this trait in a
predictable pattern, with
3 out of 4 having tall stem
height.
Probability and Heredity
• Each time Mendel
repeated the cross, he
would obtain similar
results
• He could say that the
probability, or chance,
of the cross producing a
tall plant was 3 in 4 and
the probability of
producing a short plant
was 1 in 4.
Mendel noticed the same pattern of
inheritance in other traits as well
Probability and Punnett Squares
• Punnett Square – A chart that shows how parents’
alleles might combine in an offspring
– a tool that can help you understand the patterns of
heredity
– Geneticists use Punnett Squares to show all the possible
outcomes of a genetic cross and to determine the
probability of a particular outcome
– Punnett Squares Explained at
http://www.siskiyous.edu/class/bio1/genetics/monohybri
d_v2.html
Probability and Punnett Squares
• What is the probabilty
of…..
– The offspring having a
yellow seed color?
2 in 4
– The offspring having a
green seed color?
2 in 4
Probability and Punnett Squares
• How else can
probability be written?
– The offspring having a
yellow seed color?
2 in 4 or 50%
– The offspring having a
green seed color?
2 in 4 or 50%
Using a Punnett Square
• In rabbits, black fur
color is dominant to
white. What is the
probability of
producing a white
rabbit if two
heterozygous rabbits
mate?
Using a Punnett Square
• Step 1: figure out the
genotype of the parents.
• Black is dominant to white.
– B - black
– b - white
• Parents are heterozygous.
– Bb - dad
– Bb - mom
Using a Punnett Square
• Step 2: set up a Punnett
Square.
B
B
b
b
Bb
Bb
Using a Punnett Square
• Step 3: Fill in the Punnett
Square.
B
B
b
b
Using a Punnett Square
• Step 3: Fill in the Punnett
Square.
B
B
b
BB
b
Using a Punnett Square
• Step 3: Fill in the Punnett
Square.
B
B
b
BB
b
Bb
Using a Punnett Square
• Step 3: Fill in the Punnett
Square.
B
B
BB
b Bb
b
Bb
Using a Punnett Square
• Step 3: Fill in the Punnett
Square.
B
b
B
BB
Bb
b
Bb
bb
Using a Punnett Square
• Step 4: Count the
results.
B
b
B
BB
Bb
b
Bb
bb
• Genotypes
– 1 homozygous dominant
Using a Punnett Square
• Step 4: Count the
results.
B
b
B
BB
Bb
b
Bb
bb
• Genotypes
– 1 homozygous dominant
– 2 heterozygotes
Using Punnett Square
• Step 4: Count the
results.
B
b
B
BB
Bb
b
Bb
bb
• Genotypes
– 1 homozygous dominant
– 2 heterozygotes
– 1 homozygous recessive
Using a Punnett Square
• Step 4: Count the
results.
B
b
B
BB
Bb
b
Bb
bb
• Genotypes
– 1 homozygous dominant
– 2 heterozygotes
– 1 homozygous recessive
• Phenotypes
Using a Punnett Square
• Step 4: Count the
results.
B
b
B
BB
Bb
b
Bb
bb
• Genotypes
– 1 homozygous dominant
– 2 heterozygotes
– 1 homozygous recessive
• Phenotypes
– 3 black fur
Using a Punnett Square
• Step 4: Count the
results.
B
b
B
BB
Bb
b
Bb
bb
• Genotypes
– 1 homozygous dominant
– 2 heterozygotes
– 1 homozygous recessive
• Phenotypes
– 3 black fur
– 1 white fur
Probability and Punnett Squares
• In rabbits, black fur color
is dominant to white.
What is the probability of
producing a white rabbit
if two heterozygous
rabbits mate?
– 1 in 4 or 25%
Representing Ratios
• Probability can also be represented in ratios
– A ratio compares or shows the relationship
between a part to the whole
• Example the probability that a coin will land on heads
in a single coin toss is 1 in 2 or 50%
– As a ratio, the probability would be written as 1:2
and would be read as, “One to two.”
– In genetics we use ratios to represent probability
Using a Punnett Square
• Step 4: Count the
results.
B
b
B
BB
Bb
b
Bb
bb
• Genotypes =
–
–
–
–
1 homozygous dominant
2 heterozygotes
1 homozygous recessive
Genotypic Ratio = 1:2:1
• Phenotypes
– 3 black fur
– 1 white fur
Using a Punnett Square
• Step 4: Count the results.
B
b
B
BB
Bb
b
Bb
bb
• Genotypes =
–
–
–
–
1 homozygous dominant
2 heterozygotes
1 homozygous recessive
Genotypic Ratio = 1:2:1
• Phenotypes
– 3 black fur
– 1 white fur
– Phenotypic Ratio = 3:1
Let’s Practice
1. In pine trees, long needles
are dominant over short
needles. Cross a
homozygous dominant
plant with a plant that is
heterozygous. What is the
probability that the
parents will produce
offspring that have short
needles? Give the
genotypic and phenotypic
ratios.
Let’s Practice
2. In cats, a striped coat is
dominant over a solid coat.
Cross a cat with a solid coat
with a cat that is
heterozygous for a striped
coat. What is the
probability that the parents
will produce offspring with
a solid coat? Give the
genotypic and phenotypic
ratios.
What does pedigree mean?
Pedigree: a diagram that traces one trait
through several generations of a family
= female
= male
Example #1
3
How many girls? ___
2
How many boys? ___
mom
daughter #1
dad
daughter #2
son
mom
dad
oldest
daughter #1
youngest
daughter #2
son
A horizontal line connecting two
shapes represents a cross/marriage.
mom
dad
Example #2
How many crosses/marriages?
2
A
C
B
D
E
F
G
A vertical line extending down from a
marriage/cross represents that the
couple has children.
mom
dad
A
C
B
D
questions…
E
F
G
Example #3
A
C
How many couples
have children?
B
D
H
G
E
F
M
J
N
P
K
L
O
Q
?
R
S
T
4
How do you show twins?
A
C
B
D
E
F
How do you show traits? Shading
( shaded = recessive )
RR
Rr
rr
Mutation/
Deceased
**A pedigree chart follows one trait.
Rr
mom
RR
dad
R = tongue roller
r = non-roller
Rr
son
Rr
daughter
RR
daughter
R
R
R
RR
RR
r
Rr
Rr
A
C
B
D
H
H
G
E
F
M
J
N
P
K
L
O
Q
R
S
T
Interpreting a Pedigree
• What can you tell from a pedigree?
– Whether a family has an autosomal or sex-linked
disease or disorder
• Autosomal disorder: appears in both sexes equally
• Sex-linked (X-linked)disorder: allele is located only on
the X or Y chromosome. Most sex-linked genes are on
the X chromosome and are recessive
• So who would have an X-linked disorder more often,
boys or girls?
– Whether a disorder is dominant or recessive
Sex determination
Sex chromosomes – determines the
sex of an individual
Y
X
Males have X and Y
Two kinds of gametes
Female have two X’s
Only one type of gamete
Which parents determines the sex of the offspring?
XY
male
X
XX
Y
X
Female
X
It is the male that determines the sex of
the offspring.
Sex Linked Traits
Traits controlled by genes located on
sex chromosomes (X or Y)
Most are X-linked
The X chromosome is larger and has more
genes
Y has very few genes
Show inheritance patterns that differs
for autosomal traits.
X-linked Inheritance
Males only have one copy of the X
chromosome (hemizygous), plus a Y
Female have two X chromosomes, and
may be homozygous or heterozygous for
a trait
X-linked genes are never passed from
father to son. The Y chromosome is the
only sex chromosome that passes from
father to son.
Females with one copy of the normal
gene and one copy of the mutated
gene are called carriers. They don’t
show the trait.
Males are never carriers – if they
have a mutated gene on the X
chromosome, it will be expressed
X-linked disorders
 Recessive
 Red-green color
blindness
 Hemophilia
 Dominant
 Hypertrichosis
Red-green color blindness
Can not distinguish red from green.
Normal redgreen
Red-green
colorblind
Sickle Cell Anemia
 Disorder where abnormal
hemoglobin (a protein
inside red blood cells) is
produced and warps red
blood cells
 Sickle cells deliver less
oxygen to body’s tissues and
can get stuck in small blood
vessels
 Autosomal Recessive trait,
tends to be seen in people of
African or Mediterranean
descent
Cystic Fibrosis
 Recessive, autosomal disease
 Life threatening, causes thick mucus to build up in
various areas of the body (lungs, digestive tract, etc).
 Tends to run in Caucasians, of Northern/Central European
descent
(1 in 29 Americans
carry the allele)
 Average life span in US
for people with CF is 37,
death usually caused by
lung complications
Hemophilia
 Bleeding disorder, where
it takes a long time for
blood to clot (body lacks
proteins involved in
clotting)
 Recessive, Sex-linked
(carried on the X chromosome)
 Treatment
involves
injection with
missing clotting
protein
Make a cross with an X-linked gene
Hemophilia is an
X-linked
________________
recessive
________________
disease
H for normal dominant blood
Use ______
clotting gene.
h for recessive hemophilia gene.
Use ______
h
X
On X chromosome so write it as ________
Mother
H H
Without hemophilia = X X
H
Without hemophilia = X X
h
With hemophilia = X X
h
h
Unaffected
Carrier
Affected
Father
H
Without hemophilia = X Y
h
With hemophilia = X Y
Unaffected
Affected
Make a cross with an X-linked gene
H
X
H
X
h
X
Y
H
H
X X
H
h
X X
H
X Y
h
X Y
Carrier Mom X
Normal dad
GIRLS
50% = normal
_______
50% = look normal
_______
but are
CARRIERS
BOYS
50% = normal
_______
_______
50% = hemophilia
Hemophilia
Incomplete Dominance
Codominance
Multiple Alleles
In a nutshell!
Dominant/Recessive
• One allele is dominant
over the other
(capable of masking
the recessive allele)
PP = purple
pp = white
Pp = purple
Dominant/Recessive
• In pea plants, purple flowers (P) are dominant over
white flowers (p) show the cross between two
heterozygous plants.
GENOTYPES:
- PP (1); Pp (2); pp (1)
- ratio 1:2:1
PHENOTYPES:
- purple (3); white (1)
- ratio 3:1
P
p
P
PP
Pp
p
Pp
pp
Incomplete Dominance
• A third (new) phenotype appears in the
heterozygous condition.
• Flower Color in 4 O’clocks
RR = red
rr = white
Rr = Pink
Incomplete Dominance
• In Incomplete Dominance, every genotype has its own
phenotype. (One allele not completely dominant over the other.)
Third phenotype that is a blending of the parental traits. (2
alleles produce 3 phenotypes.)
– Result: Heterozygous phenotype somewhere in between
homozygous phenotype.
Problem: Incomplete Dominance
• Show the cross between a pink and a white
flower.
GENOTYPES:
- Rr (2); rr (2)
- ratio 1:1
PHENOTYPES:
- pink (2); white (2)
- ratio 1:1
R
r
r
Rr
rr
r
Rr
rr
Codominance
In codominance, neither alleles are dominant; both
are expressed. A cross between organisms with
two different phenotypes produces offspring with
has both phenotypes of the parental traits shown.
Codominance
• Both alleles contribute to the phenotype.
– Example: In some chickens
Black Chicken x White  Speckled Chicken
YOU tell me which type of
dominance…
Codominance!
Type of
Dominance?
Incomplete
Dominance!
Type of Dominance?
Incomplete Dominance!
Multiple Alleles
• There are more than two alleles for a
trait
• Example: Blood type in humans
• Blood Types? (phenotypes)
– Type A, Type B, Type AB, Type O
• Three Blood Alleles
– A, B, O ( IA, IB, i)
Rules for Blood Type
(Genotypes)
• A and B are codominant
– AA = Type A
– BB = Type B
– AB = Type AB
• A and B are dominant over O
– AO = type A
– BO = type B
– OO = type O
• The alleles are as follows:
ALLELE
IA
IB
i
CODES FOR
Type "A" Blood
Type "B" Blood
Type "O" Blood
• According to the symbols used in the table below, that
the allele for "O" (i) is recessive to the alleles for "A" &
"B".
• With three alleles we have a higher number of possible
combinations in creating a genotype.
GENOTYPES
RESULTING PHENOTYPES
IAI A
Type A
IAi
Type A
IBIB
Type B
IBi
Type B
I AI B
Type AB
ii
Type O
Problem: Multiple Alleles
• Show the cross between a mother who has
type O blood and a father who has type AB
blood.
O
O
GENOTYPES:
- AO (2) BO (2)
- ratio 1:1
A
AO
AO
PHENOTYPES:
- type A (2); type B (2)
- ratio 1:1
B
BO
BO
Problem: Multiple Alleles
• Show the cross between a mother who is
heterozygous for type B blood and a father who is
heterozygous for type A blood.
GENOTYPES:
-AB (1); BO (1);
AO (1); OO (1)
- ratio 1:1:1:1
PHENOTYPES:
-type AB (1); type B (1)
type A (1); type O (1)
- ratio 1:1:1:1
A
O
B
AB
BO
O
AO
OO
Sample Problem
A woman with Type O blood and a man who is
Type AB are expecting a child. What are the
possible blood types of the kid?
Solve this using the symbols for blood type alleles & the good old
punnett square.
1. Figure out the genotypes of mom & dad using the given info.
2. "Woman with Type O" must be ii, because that is the one & only genotype for
Type O.
3. "Man who is AB" must be IAIB, again because it is the one & only genotype for
AB blood.
4. So our cross is: ii x IAIB. Make a punnett square
As you can see, our results are as follows:
50% of kids will be heterozygous with blood Type A 50%
will be heterozygous with blood Type B
Multiple Alleles vs. Polygenic
• Polygenic = is the inheritance pattern of a trait
that is controlled by two or more genes. Each
gene may have two or more alleles
• The resulting phenotype usually shows a
range of value for the trait. (ie. human height
or human hair color)
Genetics
IF:
Prince Charming
is blond
What color hair
should their children
have?
Snow White
has dark hair
First you must understand:
There are three basic kinds of genes:
– Dominant - A gene that is always expressed and
hides others
– Recessive - A gene that is only expressed when a
dominant gene isn’t present
– Codominant - Genes that work together to produce
a third trait
Genetics
Dominant and Recessive Genes
• A dominant gene will always
mask a recessive gene.
• A “widows peak” is dominant,
not having a widows peak is
recessive.
• If one parent contributes a
gene for a widows peak, and the
other parent doesn’t, the offspring will have a widows peak.
Widows Peak
Genetics
Punnet Square - A tool we use for predicting the
traits of an offspring
– Letters are used as symbols to designate genes
– Capital letters are used for dominant genes
– Lower case letters are used for
recessive genes
– Genes always exist in pairs
Genetics
A Widows Peak, dominant, would be symbolized with
a capital “W”, while no widows peak, recessive,
would be symbolized with a lower case “w”.
Father - No Widows Peak - w
Mother - Has a Widows Peak - W
Genetics
All organisms have two copies of each gene, one
contributed by the father, the other contributed by
the mother.
Homozygous - Two copies of the same gene
Heterozygous - Two different genes
Genetics
For the widows peak:
WW - has a widows peak
Ww - has a widows peak
ww - no widows peak
Homozygous dominant
Heterozygous
Homozygous recessive
Genetics
Since Herman has no widows peak, he must
be “ww”, since Lilly has a widows peak she
could be either “WW” or “Ww”
ww
Either Ww
or WW
Homozygous
recessive
Heterozygous
Homozygous
dominant
Genetics
We can use a “Punnet Square” to determine
what pairs of genes Lilly has
• A Punnet Square begins
with a box 2 x 2
Assume Lilly is heterozygous
Ww
Assume Herman is homoozygous
recessive
ww
• One gene is called an
“allele”
W
w
w
Ww ww
w
Ww ww
• One parents pair is split
into alleles on top, the
other along the side
• Each allele is crossed
with the other allele to
predict the traits of the
offspring
Genetics
Notice that when Lilly is crossed with
Herman, we would predict that half the
offspring would be “Ww”, the other half
would be “ww”
Half “Ww”, Heterozygous, and will
have a widows peak
Half “ww”, Homozygous, and
not have a widows peak
W
w
w
Ww will
ww
w
Ww ww
Genetics
Another possibility is that Lilly might be
“WW”, homozygous dominant.
Assume Lilly is homozygous
dominant
WW
W
Assume Herman is homoozygous
ww
W
w
Ww Ww
w
Ww Ww
Notice that all the
offspring are
heterozygous and will
have a widows peak
Genetics
So which is true? Is Lilly homozygous dominant
(WW) or is she heterozygous (Ww)?
W
w
W
W
w
Ww ww
w
Ww Ww
w
Ww ww
w
Ww Ww
Genetics
If Lilly were heterozygous,
then 1/2 of their offspring
should have a widows peak, 1/2
shouldn’t
W
If Lilly were homozygous, all
of their children will have a
widows peak
w
W
W
w
Ww ww
w
Ww Ww
w
Ww ww
w
Ww Ww
Genetics
Recall that Herman and Lilly had another
offspring, Marylin. She had no widows peak,
therefore, Lilly must be heterozygous.
Genetics
So, back to the original question. What color
hair will the offspring of Prince Charming and
Snow White have?
Genetics
Hair color is different from widows peak, no
color is truly dominant.
– Brown and blond are the two, true traits
– Homozygous conditions produce either brown or
blond hair
– Heterozygous conditions produce red hair
Genetics
For Snow White to have brown hair she must be
homozygous dominant, “BB”, a blond Prince
Charmin must be homozygous recessive, “bb”.
B
B
b
Bb
Bb
b
Bb
Bb
Genetics
All the offspring from Prince Charming and
Snow White will therefore be heterozygous,
“Bb”, and since hair color is codominant…..
all their children will have red hair.
+
Are you the Dominant or Recessive
type?
Dominant: overpowers the recessive; will be
expressed if one or both genes in the genotype
is/are dominant
Recessive: is masked by the dominant; will
only be expressed if both genes in the genotype
are recessive
Tongue Rolling
Recessive = non-roller
Dominant = roller
RR or
Rr
rr
Ear Lobes
Dominant = free
Recessive = attached
EE or
Ee
ee
Hairline
Dominant = widow’s
peak
Recessive = straight
AA or
Aa
aa
Hair Color
Dominant = Non-red
Recessive = Red
DD or
Dd
dd
Dimples
Dominant = Dimples
Recessive = no
dimples
II or
Ii
ii
Hair
Dominant = Curly/wavy
Recessive = straight
NN or
Nn
nn
Eyelashes
Dominant = Long
Recessive = Short
GG or
Gg
gg